In many signal processing applications, such as radio transceivers, it is necessary to convert a signal with an unknown amplitude into a signal which has an amplitude at a desired level. This amplitude control or "leveling," as it is sometimes referred to hereafter, is performed for a variety of reasons including for example:
to provide a constant drive level for frequency mixers for phase comparators PA1 to ensure that an amplifier input is not overdriven PA1 to remove unwanted amplitude noise from a signal.
Traditionally, amplitude leveling is performed using two different types of circuits: limiters and automatic gain control (AGC) amplifiers with peak detection. As will be explained below, each of these circuits suffers drawbacks.
Limiters provide a large signal gain and produce a constant output level by "clipping," i.e., limiting, the signal peaks of input signals. Consequently, if a sinusoidal type waveform is received at the input of the limiter, the limiting amplifier produces a square wave output "clipping off" the positive and negative peaks of the sinusoid. Limiter circuits are often employed in the back end of an FM receiver to remove AM information from the received signal.
A significant drawback of limiters is their non-linearity. In the sinusoidal input example, the amplitude is effectively limited to a desired value but at great cost. The input sinusoidal waveform is distorted, and the output is more like a square wave than a sinusoid. Accordingly, limiter circuits are not appropriate in applications where the linearity and waveform shape of the input signal must be preserved at the output. Examples of such applications include a receiver chain prior to out-of-band filtering, and a linear transmitter where harmonic levels must be controlled, or any signal processing performed on signals with amplitude modulation.
The other category of traditional amplitude levelers is AGC amplifiers with peak detection. AGC amplifiers use feedback in an attempt to maintain signal wave shape as amplitude level is controlled. However, AGC amplifiers with peak detection detect either the negative or the positive peak of the signal rather than detecting the peak-to-peak level or the RMS level of the input signal. As a result, AGC levelers are not accurate for non-symmetrical signals.
There are other drawbacks with an AGC approach. AGC amplifiers with peak detection also do not operate on differential signals. This is a substantial problem for integrated circuit (IC) applications in which differential signals are commonly employed. Still further, AGC amplifiers with peak detection require a large capacitor to hold the peak value of the detected signal. Typically, such a capacitor is too large to be integrated into solid state form, and therefore, it must be provided as a discrete component increasing the size and manufacturing costs of the amplitude limiting circuitry.